Wind and Sea

Formation and Persistence of Micro-scale Gas Pockets on Superhydrophobic Surfaces

Tait Pottebaum

Our experiments examine the formation and stability of micro-scale gas pockets on superhydrophobic surfaces (SHS), in particular in presence of steady and fluctuating shear and pressure forcing. Important micro-scale transport processes include dynamic contact lines of the gas nuclei, the influence of gas diffusion, and the changes in surface energy due to the presence of soluble surface-active agents in the flow. A goal will be a more complete physical understanding of the mechanisms that govern the formation of micro gas pockets on SHS, the processes that can promote the Cassie-Baxter wetting regime over the Wenzel wetting state, and the factors that can result in the breakdown between these two states. Surface wettability can be manipulated through the use of coatings and nano-structured patterns. Furthermore, the in-house capability to grow carbon nanotubes (CNT) allows to tune the chemical and physical properties of CNTs in order to achieve superhydrophobicity. Another target of the study addresses the interaction of these micro-scale gas pockets with near-wall turbulent flows. To that purpose, high effective Reynolds number flows are simulated in our laboratory-scale facilities by strongly forcing a laminar boundary layer with free-stream turbulence. In that particular area, hydrodynamic wall friction reduction is sought through the use of superhydrophobic CNT carpets. Research is conducted to assess the capability of CNT carpets to retain gas pockets over long periods of time and to regenerate them. In the illustrations shown here, micro-bubbles are generated in quasi-static conditions at a wall through micro-holes, using constant flow air supply. Their formation is studied through the use of high-speed imaging.